Figure 3-4. - Typical overhead slab form.

Sheathing. Shapes and holds the concrete. Plywood (usually 3/4-inch thick) or solid sheet metal (usually corrugated) is the best for use.

Joists. Supports the sheathing against deflection. Performs the same function as STUDS in a wall form. Normally, you should use 4-inch lumber; however, 2-inch or 3-inch stock can be used if properly designed.

Stringers. Supports the joists against deflection. Performs the same function as WALES in a wall form, except stringers do not need to be doubled. Use 2-inch-thick lumber or larger.

Shores. Supports the stringers against deflection. Performs the same functions as TIES in a wall form and also supports the concrete at the desired elevation. The lumber used for this must be as large as the stringers, but never smaller than 4 by 4 inches in dimension.

Lateral bracing. May be required between adjacent shores to keep the SHORES from bending under load. Usually 1 by 6 inch or larger stock is used for bracing. Bracing of some type will always be required to support the formwork. Wedges. Are normally use for two purposes: the wedges are used for leveling of the forms and the forms are easier to strip if wedges are used.

Mudsills. Continuous timber placed on the ground that distributes a load and provides a level surfaces for scaffolding and shoring.

Design Procedures

Step 1. The engineer will design and specify the materials you will need to construct the overhead forms. Ensure that all the correct materials are on the jobsite and your crew is familiar with the materials and structural members.

Step 2. Determine the maximum total load the forms will have to support. The rule of thumb for figuring the total load is live load (LL) plus dead load (DL). The live load (materials, personnel, and equipment) is estimated to be 50 pounds per square foot unless the forms will support engine-powered equipment. In this case, the LL would be 75 pounds per square foot. The dead load (concrete/rebar) is estimated at 150 pounds per cubic foot. However, you cannot add dead load to live load until you convert the dead load to square feet (SF). The formulas are as follows:

For example, if the slab is 6 inches in thickness, the formula would be as follows:

Step 3. Determine the maximum joist spacing. Use table 3-1 and read the joist spacing based on the sheathing material. Use the maximum TL in place of the maximum concrete pressure. For example, the sheathing is 3/4-inch plywood (strong way), the TL is 150 pounds per square foot, and the joist spacing would be 22 inches.

Step 4. Calculate the uniform load on the joist. The same procedure is used as for determining uniform loads on the structural members in the wall form design.

Step 5. Determine the maximum stringer spacing. Use table 3-2 and the uniform load on the joist is calculated in Step 4. Round this load up to the next